LeClanche cells have been commercially important for over a century, and in existence for more than 120 years. Within the last twenty years the commercial importance of LeClanche cells has diminished as a result of competition from alkaline cells, which provide longer life and generally superior performance. Alkaline cells cost two to four times more than LeClanche cells, however. Despite the commercial success and performance advantages of alkaline cells, however, LeClanche cells currently command an 18% share of the U.S. consumer round cell market. In Japan and most Third World countries LeClanche cells command a larger share of the consumer round cell market. Thus, LeClanche cells continue to command a commercially important segment of the worldwide consumer battery market, and are likely to do so for the foreseeable future.
Most improvements in the capacity and shelf life of LeClanche cells occurred between 1945 and 1965 before alkaline primary cells became commercially important. During those years new materials such as beneficiated manganese dioxide and zinc chloride electrolyte, and new designs such as paper lined cells, were introduced. Since 1965, however, few significant improvements in the performance of LeClanche cells have been made. Instead, over the past thirty years most changes in the design, construction and materials of LeClanche cells have been related to attempts to reduce mercury concentrations, corrosion of the zinc can anode, and hydrogen gas evolution.
To understand how LeClanche cell technology has evolved, it is helpful to review the basic function, components, and structure of such cells. LeClanche cells have a chloride-based electrolyte usually comprising a mixture of zinc chloride, water, ammonium chloride, sometimes zinc oxide, and optionally other pH-controlling materials or organic corrosion inhibitors. The cathode of a LeClanche cell typically comprises a mixture of manganese dioxide powder or granules, carbon or graphite particles, and the foregoing electrolyte mixture which at least partially wets the cathode mixture. In a LeClanche cell, a carbon rod, or pencil, is typically centrally disposed in a metal container comprising zinc, wherein the container (or can) functions as an anode, a negative current collector, and as a container in which various other elements of the cell are disposed. The carbon rod functions as a positive current collector, and is surrounded by the at least partially wetted cathode mix, which, in turn, engages the inner surface of the separator along the cathode's outer periphery. The separator is disposed between the outer periphery of the Wetted cathode mix and the inner surface of the zinc container, or anode. The electrolyte permeates the cathode mix and the separator, and permits ionic transfer to occur between the anodic zinc can and the MnO.sub.2 particles contained in the cathode.
Because the pH of a LeClanche cell is acidic, the chloride-based electrolyte strongly promotes the parasitic corrosion of zinc at the boundary between the inner surface of the can and the electrolyte. In fact, the zinc anode in a LeClanche cell is typically consumed by such reactions to such an extent that by the end of the cell's useful storage life, corrosion is visually apparent and the walls of the can are noticeably thinner. Such parasitic corrosive reactions not only affect the structural integrity of the can but, more importantly, often reduce significantly the capacity (and therefore the performance) of a LeClanche cell when it has been in storage prior to use. The effect is more pronounced at high temperatures, where even more capacity is lost in storage due to such parasitic corrosion reactions.
The basic reactions governing corrosion of a zinc can in a LeClanche cell are as follows: EQU 2H.sub.2 O+2e.fwdarw.H.sub.2 +2(OH) (eq. 1) EQU Zn.fwdarw.Zn.sup.+2 +2e (eq. 2)
Equation 1 describes the cathodic reduction of water at the inner surface of the zinc can. Equation 2 describes the oxidation of metallic zinc to valence state +2, wherein two electrons are released.
The two reactions are related in that the onset of one reaction induces the occurrence of the other, and thus induces the continuation or perpetuation of both reactions. The two interrelated reactions are not desired because they corrode the zinc can, and because they increase the amount of hydrogen gas present inside the sealed cell.
Equation 1 shows that the zinc metal of the can and water in the electrolyte of a LeClanche cell typically react to form hydrogen gas, which accumulates inside the cell. Some provision must be made for permitting the egress of such evolved gas to avoid cell rupture. Rupture of a LeClanche cell typically involves not only the release of hydrogen gas, but also the release of cathode mix containing acidic, corrosive electrolyte which can harm the device containing the cell. Carbon rods used in most LeClanche cells are often slightly porous and permeable, and therefore permit the egress of a nominal amount of evolved hydrogen gas from the cell interior. Because such carbon rods are often impregnated with wax and therefore cannot permit the egress of substantial amounts of evolved hydrogen gas, however, some allowance must typically be made in the design of LeClanche cell seals and containers for increased cell internal pressure owing to the accumulation of hydrogen gas therewithin. But excessive hydrogen gas production can lead to seal failure through over pressurization beyond the gas venting limits of the seal. Venting degrades the seal and allows water vapor to escape from the cell, resulting in cell dehydration and failure. Venting also typically permits oxygen to enter the cell, where it accelerates the aforementioned corrosion reaction at the inner surface of the zinc can by reacting directly with the zinc.
Equation 2 describes the basic corrosion reaction that typically occurs in LeClanche cells, wherein the zinc can progressively dissolves or corrodes, causing the walls of the zinc can to thin. Additionally, premature structural failure of the battery may occur through localized corrosion or "pinholing." Excessive corrosion can also cause premature performance failure of the battery through loss of ionic transport contact between the zinc can and the separator.
Corrosion of the zinc can in a LeClanche cell actually results from three different reactions:
corrosion of the zinc can occurring during the generation of electricity by the battery; PA1 parasitic corrosion of the zinc can occurring during discharge of the battery, and PA1 parasitic corrosion of the zinc can occurring when the battery is in storage and is not being discharged. PA1 add inorganic corrosion inhibitors to the cathode mix; PA1 add organic corrosion inhibitors to the cathode mix, and PA1 make zinc cans from alloys containing a mixture of zinc, lead, cadmium, manganese, or other metals that inhibit parasitic corrosion reactions. PA1 [M]etallic impurities such as copper, nickel, iron, and cobalt cause corrosive reactions with the zinc and must be avoided. In addition, iron makes zinc harder and less workable.
The first of the foregoing corrosion reactions fulfills the intended function of the battery, e.g. the generation of electricity, and thus should not be hindered. The second and third of the foregoing corrosion reactions, however, actually reduce the capacity of the battery, and thus should be prevented to the greatest degree possible.
Various solutions to the gassing and corrosion problems attending LeClanche cells have been sought for decades. The most popular and widely employed solutions to both problems in LeClanche cells have been to:
Several prior disclosures have been made suggesting the foregoing attempts to solve the corrosion and gassing problems characteristic of LeClanche cells, including:
______________________________________ Patent Inventor/Applicant/ Country Number Publisher Issue Date ______________________________________ U.K. -- Aufenast et al. 1963 U.K. -- Shreir 1963 U.S.A. 3,650,825 Lihl 1972 U.S.A. 3,877,993 Davis 1975 U.S.A. 3,928,074 Jung et al. 1975 U.S.A. 3,970,476 Cerfon 1976 U.S.A. -- Linden 1984 Japan -- Miyazaki et al. 1987 Japan -- Nikkei New 1992 Materials Belgium -- Meeus 1993 ______________________________________
In the proceedings of the 3rd International Symposium for Research and Development in Non-Mechanical Electrical Power Sources held at Bournemouth, the United Kingdom in October, 1963, subsequently published in 1963 by the MacMillan Company of New York in Volume 1 of the compilation "Batteries," in the article "Gas formation in dry cells," Aufenast and Muller discuss gas evolution and zinc corrosion in LeClanche cells at pp. 335-355. They disclose that undesired hydrogen gas production resulting from corrosion of the zinc can of LeClanche cells depends on the quality of the zinc can, on the composition of the electrolyte, and on small quantities of impurities. Aufenast and Muller disclose experiments wherein zinc strips having varying concentrations of different metal impurities were submerged in an electrolyte containing water, ammonium chloride, and zinc chloride. The hydrogen gas developed by each bimetallic couple was then measured over a fixed length of time. Their discussion on page 340 points out that ferrous iron and zinc produce "moderately active" hydrogen gas evolution, and that ferric iron shows no hydrogen gas activity at all.
In the foregoing compilation "Batteries," Shreir shows at pp. 195 that when a bimetallic couple of zinc and iron is placed in a solution of water and 1% NaCl, significant weight loss, or corrosion, of the zinc occurs, whereas the iron remains essentially uncorroded.
In U.S. Pat. No. 3,650,825 Lihl discloses a method of manufacturing an improved electrical contact by treating a known contact material such as silver or copper with mercury to enhance the electrical conductivity and contact making properties of the contact.
For many years mercury has remained the most popular and widely used of the inorganic corrosion inhibitors despite its relatively high cost. Mercury is, however, highly toxic. Almost all LeClanche cells are typically disposed of by being thrown away along with ordinary household garbage and trash, whereupon they enter the ordinary waste stream. While individual LeClanche cells usually contain only a small amount of mercury, the cumulative effect of large numbers of mercury-containing LeClanche cells entering the waste stream could cause significant quantities of mercury to be released to the environment.
Because mercury is toxic, numerous other inorganic and organic corrosion inhibitors, including various petroleum-based products, mineral oils, animal oils, chromates, and chromic acids, have been tested or used in LeClanche cells. Most such inhibitors, however, do not permit the total elimination of mercury from LeClanche cells. Instead, they typically permit only a reduced amount of mercury to be used, and do not permit the total elimination of mercury from LeClanche cells.
In U.S. Pat. No. 3,877,993 Davis discloses a LeClanche cell having an organic corrosion inhibitor comprising polymerized or copolymerized dimethyl diallyl quaternary ammonium salt. Davis' corrosion inhibitor disperses through the cathode mixture via the electrolyte to the inner surface of the zinc can to be deposited on the inner surface of the zinc can anode where it inhibits, to some degree, the aforementioned corrosion and gassing reactions. Davis' corrosion inhibitor enables the amount of mercury required in a LeClanche cell to be lowered.
In U.S. Pat. No. 3,928,074 Jung et al. disclose a LeClanche cell having a polyethylene glycol monoalkyl ether (PEL) corrosion inhibitor added to the ammonium chloride/water electrolyte thereof. The organic PEL additive reduces gassing rates in LeClanche cells having no mercury to levels commensurate with similarly constructed LeClanche cells containing mercury.
In U.S. Pat. No. 3,970,476 Cerfon discloses a LeClanche cell having a mixture of electrolyte and an organic ascorbic acid corrosion inhibitor. Cerfon discloses superior high temperature storage characteristics resulting from the addition of ascorbic acid to the ammonium chloride/water electrolyte of a LeClanche cell.
Another means of attempting to solve the gassing and corrosion problems attending LeClanche cells has been to form the zinc cans thereof from alloys containing a mixture of zinc, lead, and cadmium, wherein the inner wall of the can is coated with an amalgam of mercury. Cadmium is typically included in such zinc can alloys because it aids the zinc can manufacturing process. Typically, about 0.01% by weight mercury is added to the electrolyte of the LeClanche cell at the time of cell manufacture in the form of mercurous chloride. After the cell is assembled and closed, the mercury disperses towards the inner walls of the zinc can to form a protective mercury-zinc amalgam thereon. The mercury-zinc amalgam reduces undesired parasitic corrosion and gas evolution reactions in LeClanche cells.
In the book entitled "Handbook of Batteries and Fuel Cells," published in 1984 by McGraw-Hill Publishing Company, Chapter 5 of which is hereby incorporated by reference, at pp. 5-7 Linden discloses LeClanche cells having zinc cans containing up to about 3000 ppm cadmium and more than 3000 ppm lead. Linden discloses further that lead contributes to the forming qualities of the can, that cadmium makes the zinc corrosion-resistant to ordinary dry cell electrolytes, adds strength to the can, and is usually present in amounts of up to 1000 ppm. At page 5-7 Linden states that:
In the paper "New alloy composition for zinc can for carbon-zinc dry cells," published in 1987 by the JEC Press in vol. 6 of "Progress in Batteries & Solar Cells," which paper is hereby incorporated by reference, at pp. 110-112 Miyazaki et al. disclose LeClanche cells having no mercury therein, wherein the zinc can alloy contains a mixture of zinc, lead, cadmium, indium, and manganese, and wherein zero-mercury cells having zinc cans made of the disclosed alloy exhibit reasonably good performance characteristics and corrosion resistance in respect of LeClanche cells containing mercury.
In the paper "Mercury free dry battery materialized in Japan, mercury function substituted by a combination of materials," published in "Nikkei New Materials" in 1992, the reduction of hydrogen gassing rates through the removal of impurities from zinc can anodes in "manganese dry batteries" is disclosed at pp. 1-10.
In the paper entitled "The PMA Alloy," published in 1993 by the JEC Press in No. 5 of the "JEC Battery Newsletter," which paper is hereby incorporated by reference, Meeus discloses at pp. 30-43 zinc cans having lead concentrations as low as 2000 ppm, having no cadmium therein, and made by extruding zinc cans from calots. At page 33 Meeus discusses the beneficial effects of having lead concentrations in zinc cans exceeding 2000 ppm, wherein such lead concentrations reduce gassing and corrosion rates.
The foregoing means of reducing or eliminating mercury in LeClanche cells through the use of special alloys in the zinc can anode require, however, the presence of significant amounts of lead, cadmium, or both. It is well known that lead and cadmium are toxic metals. The special zinc can alloys developed to eliminate the use of mercury in LeClanche cells, and known of heretofore, do not contain reduced concentrations of either or both of those toxic metals. Some of the foregoing special alloys even contain elevated concentrations of both toxic metals.
Other efforts to improve the performance, capacity, and storage characteristics of mercury-free LeClanche cells have been directed to identifying suitable inhibitors for use in mercury-free cells.
Ihara et al., "Mechanism of Corrosion Inhibition with Bismuth Compounds for Iron, Nickel and Zinc in Acid Solutions," 60 Derki Kagaku 500-507 (1992), demonstrates the use of bismuth compounds, including organobismuth compounds for inhibiting corrosion by formation of protective films on electrodes, including zinc electrodes.
Huang, et al., Mercury-Free zinc/manganese batteries with paper separators," Chinese Patent Document CN 1065553A (1992) (Abstract translated at CA Selects Batteries & Fuel Cells, Issue 10, page 5, 1993, describes the use of paper separators coated with 0.25-0.45% BiCl.sub.3 paste to suppress corrosion of Zn anodes.
Chen, et al., "Corrosion of Ammonium Chloride Mercury-Free Paper Lined Battery During Storage," Dianchi 24:20 (1994), report certain conditions and procedures for improving corrosion resistance using Bismuth compounds in mercury free general purpose batteries.
Huang, et al., "Mercury-Substituting Inhibitors for Zinc Manganese Dry Cells," 24 Battery Monthly 260(1994) evaluated bismuth as a corrosion inhibitor.
What is needed is a mercury-free heavy duty LeClanche cell having acceptable performance, capacity, and storage characteristics that can be produced at a significantly lower cost than prior mercury-free heavy duty LeClanche cells.
It is therefore an object of the present invention to provide a heavy duty LeClanche cell having comparable or superior performance, capacity, and storage characteristics when compared to existing heavy duty LeClanche cells.
It is another object of the present invention to provide a heavy duty LeClanche cell having increased performance at lower cost.
It is still another object of the present invention to provide a LeClanche cell that presents a reduced hazard to the environment, wherein the cell may be disposed of in a landfill without presenting any significant hazard to human or other forms of life.
It is a further object yet of the present invention to provide a LeClanche cell having reduced or no mercury therein.
It is another object yet of the present invention to provide a LeClanche cell having low gassing rates.
It is still another object of the present invention to provide a LeClanche cell having reduced parasitic corrosion reactions occurring on the surface of the zinc anode thereof.
It is still another object yet of the present invention to provide methods of making separators for LeClanche cells and for incorporating the separators into LeClanche cells, wherein cells so made exhibit superior performance, capacity, and storage characteristics comparable or superior to existing cells.
It is a further object of the present invention to provide methods of making separators for LeClanche cells that present a reduced hazard to the environment.
It is a feature of the present invention to provide separators for a LeClanche cell.
It is another feature of the present invention to provide a Kraft paper separator paper for a LeClanche cell, which, when suitably coated exhibits performance, capacity and storage characteristics comparable or superior to existing separators.
It is another feature of the present invention to provide a separator for use in an improved LeClanche cell that decreases the extent of leakage during storage and increases the milliamp hours of performance, while decreasing gas evolution.
It is another feature of the present invention to provide a separator in a LeClanche cell, the separator having no mercury disposed on the surface thereof.
It is a further feature of the present invention to provide a LeClanche cell having reduced mercury therein.
It is yet another feature of the present invention to provide a LeClanche cell having no mercury therein.
It is a still further feature yet of the present invention to provide a LeClanche cell, the cathode, the electrolyte, the separator and the anode thereof containing, in combination, no more than about 0.01 percent by weight mercury.
It is a further feature of the present invention to provide a LeClanche cell, the cathode, the electrolyte, the separator and the zinc anode thereof containing, in combination, no, or substantially no, mercury.
It is a still a further feature yet of the present invention to provide a method of making a separator for forming a zinc anode of a LeClanche cell, the method including the steps of selecting a Kraft paper as a substrate, and thereafter depositing thereon a coating comprising an inorganic corrosion inhibitor, an organic corrosion inhibitor, and a binding agent.
It is an advantage of the present invention that the Kraft paper-containing separator thereof costs less to manufacture than a conventional capacitor-grade separator for a LeClanche cell.